The invention relates generally to non-invasive imaging such as single photon emission computed tomography (SPECT) imaging. More particularly, the invention relates to imaging systems configured to have an adjustable focal length from a pinhole aperture in a collimator assembly to a detector assembly.
SPECT is used for a wide variety of imaging applications, such as medical imaging. In general, SPECT systems are imaging systems that are configured to generate an image based upon the impact of photons (generated by a nuclear decay event) against a gamma-ray detector. In medical and research contexts, these detected photons may be processed to formulate an image of organs or tissues beneath the skin.
To produce an image, one or more detector assemblies may be rotated around a subject. Detector assemblies are typically comprised of various structures working together to receive and process the incoming photons. For instance, the detector assembly may utilize a scintillator assembly (e.g., large sodium iodide scintillator plates) to convert the photons into visible light for detection by an optical sensor. This scintillator assembly may be coupled by a light guide to multiple photomultiplier tubes (PMTs) or other light sensors that convert the light from the scintillator assembly into an electric signal. In addition to the scintillator assembly-PMT combination, pixilated solid-state direct conversion detectors (e.g., CZT) may also be used to generate electric signals from the impact of the photons. This electric signal can be transferred, converted, and processed by electronic modules in a data acquisition module to facilitate viewing and manipulation by clinicians.
Typically, SPECT systems further include a collimator assembly that may be attached to the front of the gamma-ray detector. In general, the collimator assembly is designed to absorb photons such that only photons traveling in certain directions impact the detector assembly. In certain instances, pinhole-aperture collimators may be used. Pinhole-aperture collimators are generally collimators with one or more small pinhole apertures therein. Photons passing through these pinhole apertures generally project an inverted image of the source onto the detector assembly.
In general, the system resolution and sensitivity is at least partially based on the focal length (i.e., the distance from a pinhole aperture to the detector assembly). For example, the image may be magnified if the distance from the source to the pinhole aperture is less than the focal length from the pinhole aperture to the detector assembly. In a similar manner, the image may be minified if the distance from the source to the pinhole aperture is greater than the focal length from the pinhole aperture to the detector assembly. However, the distance to the source may vary for each pinhole aperture in the collimator assembly. By way of example, if a collimator assembly with multiple pinhole apertures is placed around a thorax to image a heart (which is generally eccentered anterior and left), the distance from each pinhole aperture to the heart will typically vary.